The invention relates to a novel boron suboxide material and to a method for its preparation. More particularly, the present invention relates to a new boron suboxide material which is much harder than boron suboxides previously reported, and to a method for preparing the new boron suboxide material.
A great deal of research has been devoted to developing synthetic superhard materials which have hardness values approaching that of diamond. The best known of these synthetic superhard materials is cubic boron nitride (cBN). Other very hard binary and ternary compounds made from light elements such as boron, nitrogen, oxygen, aluminum, silicon and phosphorous are also known. Among these, the compounds C.sub.3 N.sub.4, BNC, BPN, B.sub.4 C and BP may be mentioned. Another of these compounds which has been reported as having high hardness values is boron suboxide.
Boron normally has a valence of 3 and reacts with oxygen to form boron oxide having the stoichiometric formula B.sub.2 O.sub.3. Under suitable conditions, boron may react with oxygen and form compounds in which boron exhibits a valence less than 3. Powders of nominal composition B.sub.3 O, B.sub.4 O, B.sub.6 O, B.sub.7 O, B.sub.8 O, B.sub.12 O, B.sub.15 O, and B.sub.18 O have been reported as being formed by reacting elemental boron (B) with boron oxide (B.sub.2 O.sub.3) under suitably high pressure and temperature conditions. For purposes of this disclosure, the term "boron suboxide" refers to boron/oxygen binary compounds wherein boron has a valence less than 3. Since different varieties of boron suboxide have been reported by others, boron suboxide will be generally designated with the chemical formula B.sub.x O.
The formation of boron suboxide (B.sub.x O) and a description of its properties have been extensively reported in the literature. Most of the reports in the literature attribute the formula B.sub.6 O or B.sub.7 O to the boron suboxide being investigated. In some cases, the boron suboxide formed and the material being investigated may 0 consist of more than one phase. In U.S. Pat. No. 3,660,031 (Holcombe, Jr., et al.), a method of preparing a boron suboxide material is disclosed. According to this patent, the boron suboxide material is formed by reducing zinc oxide with elemental boron at a temperature in the range of 1,200.degree. C.-1,500.degree. C. The boron suboxide produced by this method is reported as having the formula B.sub.7 O. It is also characterized as having an average hardness value as measured with a Vickers indentor under a 100 g load (VHN.sub.100) of 3,820 kg/mm.sup.2 and a density of about 2.60 g/cc. The material is described as highly refractory, and suitable for use on surfaces subject to abrasion, e.g., grinding wheels, drill bits, machine tools, etc., and in structures employed in high temperature applications.
In U.S. Pat. No. 3,816,586 (Goosey), a method of fabricating boron suboxide products is disclosed. According to this patent, a mixture of elemental boron and boron oxide is cold pressed in a tantalum lined metal die. After the pressure on the compacted mixture is released, it is coated with a mixture of boron nitride and boron oxide, and is subjected to a second pressing step while heating at a temperature sufficient to melt the boron oxide in the compacted mixture. This is followed by a cooling step and another hot pressing step. The boron suboxide product made by this method is reported as being a smooth, sound boron suboxide article, free of flaws and contaminants, and suitable for a variety of applications, e.g., in cutting tools, ceramic bearings, and grinding media. Upon analysis, the boron suboxide product gave 80.1 wt. % boron and 19.9 wt. % oxygen which corresponds to the stoichiometry of B.sub.6 O. It was also reported as having a density of 2.60 g/cc and a Knoop hardness under a 100 g load (KHN.sub.100) of 3000 kg/mm.sup.2.
Other reports on methods of preparing boron suboxide and the properties of this material are the following:
1. R. R. Pasternak, "Crystallographic Evidence for the Existence of B.sub.7 O", Acta Cryst. 12 (1959), 612; PA1 2. S. LaPlaca, B. Post, "The Boron Carbide Structure Type", Planseeberichte Fur Pulvermetallurgie, Bd. 9, 1961; PA1 3. H. F. Rizzo, W. C. Simmons, and H. O. Bielstein, "The Existence and Formation of the Solid B.sub.6 O", Journal of the Electrochemical Society, Jan. 1963; PA1 4. F. A. Halden, R. Sedlacek, "Growth and Evaluation of Boron Suboxide and Zirconium Dioxide Single Crystals", Stanford Research Institute, Menlo Park, Calif., Jan. 1963; PA1 5. H. Tracy Hall and Lane A. Compton, "Group IV Analogs and High Pressure, High Temperature Synthesis of B.sub.2 O", Inorg. Chem. 4 (1965) 1213; PA1 6. W. C. Simmons, "Progress and Planning Report on Boron Suboxide B.sub.6 O", Air Force Materials Laboratory, Mar., 1968; PA1 7. H. Wrerheit, P. Runow, and H. G. Leis, "On Boron-Suboxide Surface Layers and Surface States of B-Rhombohedral Boron", Phys. Stat. Sol. (a) 2, K125 (1970); PA1 8. E. V. Zubova, K. P. Burdina, "Synthesis of B.sub.6 O Under Pressure", Dokl. Akad. Nauk. SSR, 197 (5) (1971) 1055-1056; PA1 9. C. E. Holcombe, Jr., and O. J. Horne, Jr., "Preparation of Boron Suboxide, B.sub.6 O", Journal of the American Ceramic Society--Discussions and Notes, Vol. 55, No. 2 (1971) 106; PA1 10. D. R. Petrak, R. Ruh, B. F. Goosey, "Preparation and Characterization of Boron Suboxide", National Bureau of Standards Special Publication 364, Solid State Chemistry, Proceedings of 5th Materials Research Symposiums, issued Jul. 1972; PA1 11. W. H. Rhodes, A. J. DeLai, "Research on Development and Fabrication of Boron Suboxide Specimens," AVCO Corp., prepared for: Air Force Materials Laboratory, Aug., 1972; PA1 12. R. R. Petrak, Robert Ruh, and G. R. Atkins, "Mechanical Properties of Hot-Pressed Boron Suboxide and Boron", Ceramic Bulletin, Vol. 53, No. 8 (1974), 569-573; PA1 13. P. M. Bills and D. Lewis, "Non-stoichiometry of Boron Suboxide (B.sub.6 O)", Journal of the Less Common Metals, 45 (1976) 343-345; PA1 14. V. S. Makarov and Ya. A Ugai, "Thermochemical Study of Boron Suboxide B.sub.6 O", Journal of the Less Common Metals, 117 (1986), 277-281; PA1 15. C. Brodhag and F. Thevenot, "Hot Pressing of Boron Suboxide B.sub.12 O.sub.2 ", Journal of the Less-Common Metals, 117 (1986), 1-6; PA1 16. Tadashi Endo, Tsugio Sato, Masahiko Shimada, "High-pressure synthesis of B.sub.2 O with diamond-like structure", Journal of Materials Science Letters 6 (1987), 683-685; PA1 17. David Emin, "Icosahedra Boron-Rich Solids", Physics Today, Jan., 1987. PA1 18. Andrzej Badzian, "Superhard Material Comparable in Hardness to Diamond", Appl. Phys. Lett. 53(25), 19 Dec. 1988; PA1 19. W. E. Moddeman, A. R. Burke, W. C. Bowling and D. S. Foose "Surface Oxides of Boron and B.sub.12 O.sub.2 as Determined by XPS" Surface and Interface Analysis, v. 14 n5, May, 1989, p. 224-232.
All of the aforementioned patents and publications are incorporated herein by reference.
These publications and patents are in agreement that boron suboxide compounds can be produced according to the following chemical equation: EQU B+B.sub.2 O.sub.3 .fwdarw.B.sub.x O (1)
at moderately high pressures (e.g., 1,000 to 6,000 psi) and moderately high temperatures (e.g., 1,400.degree. C. to 2,200.degree. C.). The specific form of B.sub.x O actually produced depends on the process conditions and the ratio of elemental boron to boron oxide loaded into the reaction cell. Alternatively, boron suboxide compounds may be produced according to the following chemical equation: EQU B+MO.fwdarw.B.sub.x O+M (2)
wherein M=Mg or Zn, at temperatures ranging from 1200.degree. C. to 1500.degree. C. without applied pressure. Depending on the starting proportions of boron and metal oxide, the formula B.sub.6 O has been attributed most frequently to the boron suboxide compound formed, although some researchers report the formation of B.sub.7 O and other boron/oxygen compounds.
Generally, the hardness and density values reported for boron suboxide in all of the aforementioned publications have been in good agreement. Average KHN.sub.100 values of 3400 to 3600 kg/mm.sup.2 were reported by Simmons, and KHN.sub.100 values of 3400 to 3500 kg/mm.sup.2 were reported by Petrak et al. The KHN.sub.100 value reported in U.S. Pat. No. 3,816,586 (Goosey) is somewhat anomalous and this may be due to the presence of another phase in addition to B.sub.6 O. In any event, none of the researchers have reported a KHN.sub.100 value greater than 3600 kg/mm.sup.2 for boron suboxide. The densities reported by these researchers have been from 99.5 to 100% of the theoretical value. Furthermore, based on crystallographic data, most of the researchers have ascribed a rhombohedral unit cell to the boron suboxide that was produced.
While the reported hardness values are quite good, it would be desirable to produce a superhard form of boron suboxide having a hardness value which more closely approaches that of diamond which ranges from KHN of 6000 to 9000. More particularly, because the raw materials to form boron suboxide are inexpensive and the process is relatively simple, it would be desirable to produce a superhard form of boron suboxide which has an average KHN.sub.100 hardness value of at least about 3800 kg/mm.sup.2, and preferably in the range of about 4000-4500 kg/mm.sup.2.
Recent crystallographic studies of B.sub.6 O and B.sub.7 O have revealed that both possess the same crystal structure. This indicates these are the same defect phase and include all intermediate compositions. If used in a generic sense the term B.sub.6 O in this application henceforth refers to any composition between B.sub.6 O and B.sub.7 O, inclusive.